The bug
When opening the decoration tab in creative, all stained glass panes are visible except the light gray version. I expected to see the light gray version, but it didn't show up.
How to reproduce
Open the block menu (be in creative)
Open the decoration section
Scroll down to the chest, under the chest is the light gray version
Related issues
is duplicated by
Attachments
Comments


"Barely" visible.

It's light grey. It just so happens that your inventory is light grey. Sorry, WAI.
This is working as intended but it doesn't look nice.
As it is, the only thing visible is the quantity number.
It would be better if there was a special case for low/no contrast icons like the light grey glass pane. Or to use the original glass pane icon with the dye colour indicated by an inset square.
Absolutely no way to tell apart an empty slot from one which contains a light grey glass pane… I would not call that "resolved"
Change inventory cell background color so it wouldn't match with any item color or darken the background color if it lies below the item that is being showed. And yup - it's not "barely" it is completely invisible. And I can't consider that "as intended" there're a lot of options to make it work properly. Fix it please!

@unknown: This is a warning, please refrain from making such statements on the bug tracker in the future.
Bedrock Edition has these as 3D models, so if Mojang ever changes their mind and decides to fix this, that would probably be the easiest way to do it.
why can't glass panes use 3D models like Bedrock Edition? This way there would be a shadow on the light gray stained glass pane and people can see it.